Gestation is the period of time between fertilization and birth in oviparous animals. The shortest gestation period known is 12 to 13 days, shared by three marsupials: the American or Virginian opossum (Didelphis marsupialis); the rare water opossum, or yapok (Chironectes minimus) of central and northern South America; and the eastern native cat (Dasyurus viverrinus) of Australia. The young of each of these marsupials are born while still immature and complete their development in the ventral pouch of their mother. While 12 to 13 days is the average, the gestation period is sometimes as short as eight days. The longest gestation period for a mammal is that of the African elephant (Loxodonta africana) with an average of 660 days, and a maximum of 760 days.